In an organic electroluminescence (hereinafter, referred to as organic EL) display device, a light-emitting element is disposed in each pixel, and an image is displayed by independently controlling emission of the pixels. A light-emitting element has a structure in which a layer (hereinafter, referred to as an emission layer) is sandwiched between a pair of electrodes, where one electrode as an anode is distinguished from the other electrode as a cathode. In an organic EL display device, one electrode is provided in every pixel as a pixel electrode, and the other electrode is provided as a common electrode which extends over a plurality of pixels and is applied with a common potential. In an organic EL display device, emission of the pixels is controlled by applying the pixel electrode with a potential corresponding to an image.
A driving transistor is connected to a light-emitting element disposed in each pixel of a display device. When these driving transistors have variation in a threshold voltage, the variation could influence the luminance of the display device, leading to a defective display. In order to compensate the defective display due to the variation of the threshold voltage of the driving transistors, a display device which undergoes threshold compensation of the driving transistors and its driving method are disclosed in patent document 1 (Japanese patent application publication No. 2015-049335).